PROJECT SUMMARY Organismal diet has a profound impact on tissue regeneration, aging, and disease in mammals. However, the mechanisms through which diet perturbs stem and progenitor cell biology and leads to diseases such as cancer are poorly understood. With the rise of obesity in the US population?more than 1 in 3 adults are obese ?understanding the relationship between diet, stem cell biology, and cancer incidence takes on great importance. Focused on the mammalian intestine, we find that a pro-obesity high fat diet (HFD) augments the number and niche-independent function of Lgr5+ stem cells. Mechanistically, our studies indicate that a HFD induces a robust peroxisome proliferator-activated receptor delta (PPAR-d) signature in intestinal stem cells (ISCs) and progenitors (non-stem cells), and pharmacologic activation of PPAR-d recapitulates the effects that a HFD has on these cells. Furthermore, like a HFD, ex vivo treatment of intestinal organoid cultures with fatty acid constituents of the HFD enhances the self-renewal potential of these organoids in a PPAR-d dependent manner. Interestingly, both HFD- and agonist-activated PPAR-d signaling endow progenitors with the organoid-initiating capacity normally restricted to stem cells. In fact, agonist-enforced PPAR-d signaling permits these progenitors to form in vivo tumors upon loss of the tumor suppressor Apc. These observations provide a possible pathway for diet through modulating PPAR-d activation to alter not only the function of intestinal stem and progenitor cells but also their capacity to initiate tumors. Many questions remain regarding the impact of a HFD on the intestine, such as the in vivo, cell type-specific roles of PPAR-d in this process, and the identity of progenitor subsets that emerge in this diet to drive tumor development. Also, although PPAR-d is a master transcriptional regulator of genes involved in fatty acid oxidation (FAO), it is unclear whether intestinal stem and progenitor cells or tumors that arise in a HFD rely on this PPAR-d-activated FAO metabolic program for their maintenance. Specifically, we will test the hypotheses that PPAR-d mediates the in vivo effects of a HFD in ISCs and progenitors in intestinal homeostasis and tumor initiation (Aim 1); that a subset of non-stem cell progenitors acquire stemness and tumorigenic potential in a HFD and with enforced PPAR-d signaling (Aim 2); that a HFD or enforced PPAR-d signaling render ISCs, progenitors, or established tumors metabolically reliant on fatty acid oxidation for their maintenance (Aim 3).